License Assisted Access (LAA) is a technology being developed and deployed to augment Long Term Evolution (LTE) transmissions on licensed carriers with transmissions in unlicensed bands at both 3.5 GHz and 5 GHz. Specifically the frequency ranges 3550-3700 MHz and 5150-5925 MHz, or parts thereof, is potentially available for LAA for LTE (or LAA-LTE) and/or unlicensed operation. This represents a significant amount of spectrum that can be used by operators to augment their service offerings in licensed bands.
To address concerns that Wi-Fi, currently the dominant technology deployed in the 5 GHz band, may be adversely affected by LAA-LTE, current 3GPP proposals have suggested that LAA-LTE operators manage their channel utilization by setting a maximum transmitter duty cycle or ON duration, so that in environments where there are few Wi-Fi users (e.g. devices and/or Access Points (APs)), LAA-LTE will have more capacity, and in environments where there are many Wi-Fi users, LAA-LTE users will have less bandwidth. To further improve access fairness, LAA proposals also contemplate mandating the use of Listen Before Talk (LBT), a channel sharing functionality used by Wi-Fi devices, to ensures that no other device (e.g. a WiFi or LAA device) is employing the band prior to transmission.
The standardization of LAA by 3GPP is still ongoing, and in the interim, a non-standardized form of unlicensed LTE termed LTE-U is being developed. As currently defined, LTE-U does not require LBT and has a variable transmit ON duration between 4 msec and 20 msec. While they can operate in the 3.5 GHz and 5 GHz bands under a license-exempt or ISM regime, LTE-U implementations must share the unlicensed spectrum with existing mobile and other incumbent services. From that perspective, unlicensed access is still viewed as complementary and most implementations are expected to continue to rely on licensed allocations in view of the ever increasing demands for wireless broadband access.
However, one of the potential drawbacks of employing these unlicensed bands is that significant portions of them have incumbent radar operators which must be given priority. This means that any new unlicensed operation must defer to incumbent radar operators by monitoring for radar signals and ceasing to transmit in the unlicensed band for a period of at least 30 minutes when a radar signal is detected. In the 5 GHz unlicensed band for example, the frequency bands from 5250 to 5730 MHz fall within the Unlicensed National Information Interchange (U-NII)-2 and UNII-2 extension bands and as such, are subject to Dynamic Frequency Selection (DFS) requirements. In the US, the requirements for detecting and avoiding interference with radar signals in the unlicensed bands are specified by the Federal Communications Commission (FCC) under the designation Dynamic Frequency Selection (DFS). DFS detection requirements are formulated in terms of power signature, number of pulses, pulse width and pulse repetition interval as well as radio actions such as channel scanning times, and times when devices must keep off the channel when a radar signal has been detected.
Existing solutions for DFS detection have been developed for Wi-Fi applications that employ unlicensed spectrum. However to date, no specific solutions for LTE or LAA have been developed. Existing DFS implementations in Wi-Fi chipsets are not optimized to recognize LAA/LTE signals and avoid false positives for example, due to the presence of LTE signals such as the Common or Cell-specific Reference Signals (CRS), Channel State Information Reference Signals (CSI-RS), Sounding Reference Signals (SRS), Positioning Reference Signals (PRS) or (Primary or Secondary) Synchronization Signals (PSS/SSS). Furthermore, unless a significant fraction of the DFS radar pulses are detected with a defined pulse width and pulse repetition interval (PRI), Wi-Fi signals as well as LAA or LTE-U signals (e.g. from other access points or eNodeBs) could trigger a false detection. For radar type 5 this can be very challenging since the pulse width and pulse repetition interval can vary over the duration of the pulse sequence. Furthermore for LTE-U implementations that do not employ LAA-based LBT, the agreed minimum transmit on time is 20 msec. As such, it is possible that the DFS radar signal could be completely missed since there are modes or radar types for which the entire pulse sequence is less than 3 msec.
Accordingly, to address some or all of the drawbacks noted above, it would be desirable to have a radar detection scheme adapted for shared access over an unlicensed band.